The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
Many current passenger car multiple speed automatic transmissions include a combination of planetary gear sets and selectively engaged clutches and brakes which provide a plurality of forward torque or gear ratios.
Whereas three and four speed automatic transmissions were once commonplace and considered suitable to provide the requisite flexibility and performance, increasingly demanding economic and performance goals as well as consumer preference continue to encourage automatic transmission research and development. One result of this effort has been an increase in the number of available forward gears or speed ratios provided by the transmission. Six, seven and eight speed automatic transmissions are increasingly available.
Because they so closely match the power and torque curves of an engine to the vehicle speed and load, such six, seven and eight speed transmissions provide significant performance enhancements and fuel economies.
Study of these transmissions configurations, however, reveals that improvements are possible. For example, each of the selectively engaged frictional couplings, i.e., the clutches and the brakes, contribute to frictional losses, referred to as spin losses, when they are not engaged. Such spin losses are the result of relative rotation between the multiple, interleaved plates or discs of such devices. Two primary factors influence spin losses: the size or torque capacity of the clutch or brake and the speed difference across the clutch or brake. Reducing such spin losses therefore provides improved transmission efficiency which is highly desirable.